Exhaust-gas sensors featuring various designs are currently used for monitoring the emissions of internal combustion engines. To ensure this function, the exhaust-gas sensors must be checked at regular intervals, e.g., within the framework of an on-board diagnosis (OBD), with regard to their proper functioning. Specific operating states of the internal combustion engine need to be present in order to implement a few of the diagnostic functions required in this context. For example, the plausibility check of a measured oxygen concentration is carried out by a wideband lambda oxygen sensor, preferably during trailing-throttle operation of the internal combustion engine when no fuel is supplied to the internal combustion engine, since the deviation of the sensor signal from an expected value in the event of an error is highest under these circumstances.
Within the framework of new operating strategies for internal combustion engines and new technologies, the operating points of the internal combustion engine required to monitor the functioning of the exhaust-gas sensors are no longer activated at sufficient frequency. In the case of motor vehicles operated according to the start-stop method, for example, the internal combustion engine is switched off at standstill, so that the idling operating state is no longer present or present only very infrequently. In new technologies such as the hybrid drive, trailing-throttle operation is prevented for the most part.
In addition to monitoring the exhaust-gas sensors, learning functions for the different exhaust-gas sensors must be provided at certain intervals, e.g., a trailing-throttle adaptation in the case of wideband lambda oxygen sensors. The required operating state, such as trailing-throttle operation in the example mentioned, is called up separately for this purpose, even in the case of hybrid vehicles or start-stop systems. Simultaneously with these learning functions, the required diagnosis functions for the exhaust-gas sensors may be carried out as well. However, the implementation of the diagnosis function is restricted to the duration of the learning function and the frequency at which it is carried out.
It is an object of the present invention to provide an example method by which the proper functioning of exhaust-gas sensors is able to be monitored even if the operating points of the internal combustion engines required for this purpose are activated only infrequently.
It is a further object of the present invention to provide a corresponding device for implementing the method.